An optical interferometer is an optical transmission network that produces interference between different portions of optical radiation that have traveled different paths through the network. In an unbalanced optical interferometer the time delay in the two optical paths are different by an amount τ. The output intensity from such an interferometer will have a periodic dependence called fringes of the output intensity that is periodic versus the interferometer phase delay φ=2πντ, where ν is the optical frequency launched into the interferometer. Information about the path delay difference, the input optical frequency, or the input optical frequency spectrum may thus be deduced from the output interference signal.
An optical interferometer network may also contain more than one pair (or set) of paths from the input to the output port. Different pairs (or sets) of paths may then be interpreted as different interferometers. The interference caused by individual interferometers may be interrogated separately,    a. by assigning a specific range of optical wavelengths, thus employing a wavelength division multiplexing (WDM) technique,    b. by assigning a specific range of total transmission time delay to the paths associated with each interferometer, thus employing a time division multiplexing (TDM) technique,    c. or by assigning a specific combination of input and output ports to the paths associated with each interferometer, thus employing a space division multiplexing (SDM) technique. An SDM system may for instance be interrogated by using optical switches to access different combinations of input and output ports sequentially, or by splitting the optical radiation from a single interrogation source into different interferometer sub-networks, and connecting one detector to the output of each sub-network.Network interrogation employing combinations of WDM, TDM and $DM is also possible.
The visibility or amplitude of the output fringes depends on the states of polarization (SOPs) of the two interfering signals, which we will label SOP1 and SOP2. In many interferometers SOP1 and SOP2 will vary randomly with time due to changes in the input SOP or in the birefringence properties of the two optical pathways. The fringe visibility is proportional to the projection of SOP1 onto SOP2. The reduction of fringe visibility with reduced projection of the SOPS is called polarization fading, and generally causes a reduced signal to noise ratio in the interferometer readout. Especially, the situation with SOP1⊥SOP2 (orthogonal SOPs) causing total polarization fading with zero visibility should be avoided. When the fading is total, the interferometer output will not carry any information about ν or τ at all.
Several methods for reduction or elimination of the polarization fading problem are known. One known method uses Faraday rotating mirrors, as disclosed by A. D. Kersey et. al. in [“Polarisation insensitive fibre optic Michelson interferometer”, El. Lett., Vol. 27, pp. 518-19, 1991]. This method allows for a simple source and detection system, but it works only for the Michelson interferometer configuration. Furthermore, the Faraday rotating mirrors may be expensive, space consuming, and sensitive to extreme thermal, electromagnetic and other environmental conditions.
Other known methods are based on active polarization control at the input, as disclosed by A. D. Kersey et. al. in [“Optimization and Stabilization of Visibility in Interferometric Fiber-Optic Sensors Using Input-Polarization Control”, J. of Lightwave Technol., Vol. 6, pp. 1599-1609, 1998], or the use of a polarizer combined with active polarization control at the output end, as disclosed by K. H. Wanser et. al. in [“Remote polarization control for fiber-optic interferometers”, Opt. Lett., Vol. 12, pp. 217-19 1987]. In both cases the polarization controller is continuously adjusted to optimize the fringe visibility. These techniques require relatively complex systems to provide feedback signals to the polarization controller, and in systems with spatial division multiplexing (SDM) or wavelength division multiplexing (WDM) of multiple sensors, individual polarization controllers for each sensor are generally required. The polarization modulator used for the polarization control must be capable of modulating the SOP in three dimensions on the Poincare sphere. This generally implies that the polarization modulators at least must be of the “dual stage” type. Dual stage polarization modulators generally incorporate two independently adjustable birefringent elements, and are thus more complex and expensive than single stage modulators having only one adjustable birefringent element.
Still other known methods are based on modulating the input SOP between three states, as disclosed by A. D. Kersey et. al. in [“Input polarisation scanning technique for overcoming polarisation-induced signal fading in interferometric fiber sensors”, El. Lett., Vol. 24, pp. 931-33, 1988] and in [U.S. Pat. No. 4,932,783], or on the use of three detectors at the output with three polarizers that are adjusted to monitor different polarization states, as disclosed by N. J. Prigo et. al. in [“Technique for elimination of polarisation fading in fibre interferometers”, El. Lett., Vol. 20, pp. 319-20, 1984]. These techniques increase the complexity of the processing by requiring simultaneous processing of three fringe signals, especially in WDM systems where separate receivers are required for each WDM channel. They also increase the complexity of the hardware by either requiring a dual stage polarization modulator, or three detectors and polarizers.
Another known method is based on the use of a pulsed source and a compensating interferometer which incorporates a polarization maintaining coupler, a standard coupler, and a polarization modulator, as disclosed by B. Y. Kim et. al. in [U.S. Pat. No. 5,173,743]. This method may be attractive when applied to time division multiplexed (TDM) ladder sensor networks. However, the compensating interferometer increases the complexity of the interrogation, and 3 dB of source power is lost in the output coupler from the compensating interferometer. If the network is not of the ladder type, another 3 dB of optical power will be effectively lost at the detector, since only one half of the detected pulses will carry useful information. Another problem with this method is that the readout phase from the interferometer is sensitive to birefringence changes in the lead fiber between the source and the interferometers.
One sensor configuration of special interest is that of a WDM fiber-optic interferometric sensor system employing two identical wavelength selective low reflectivity fiber Bragg grating reflectors to define the two paths of each sensor interferometer. For instance, a Fabry-Perot interferometer can be formed by writing two identical Bragg gratings into the core of a single optical fiber at different locations. Different reflection bands should be dedicated to each sensor, so that information about the individual sensors can be accessed by use of a multi-wavelength source at the network input producing coherent light at each sensor wavelength, and a wavelength division multiplexer at the network output, that directs the interference signal from each sensor to a dedicated detector.